The purpose of the proposed research is to study the relationships among the RNA's of avian RNA tumor viruses and between these and endogenous RNA molecules. The close relatedness of all these viruses strongly suggests an evolutionary relationship between them and it is quite likely that all are evolved from the DNA of the host cell. Thus a study of sequence relationships in the viral RNA's should provide information essential to the understanding of RNA tumor virus evolution. The approach to be taken in this study is as follows: Endogenous "virus" RNA molecules will be isolated by nucleic acid hybridization using a specially modified DNA probe which allows the purification of molecules containing virus-specific sequences. These molecules will be compared with the RNA's of transforming, non-transforming, and spontaneous virus RNA's by nucleic acid hybridization and hybridization-competition to determine large scale differences and by RNA fingerprinting to sample the genomes for fine differences. Related and unrelated regions will be characterized in terms of which proteins they code for and their location in the genome. It is expected that this study will throw light on the molecular events of tumor virus evolution and provide information concerning the function of endogenous "virus" information.